"Star Wars" style scrolling title?

...but just something where a block of text can
scroll smoothly up from the bottom and disappear up
the top of the screen (if that makes sense).

Have you tried "Scrolling Block"?

2) Also looking for a way to make "subtitles" that
appear lower on the screen than the one provided with
iMovie.

Note that iMovie lets you adjust the vertical position of certain titles. As the title plays in the little preview window, move the mouse over the window. If the cursor changes to a cross ➕ the title is movable. Click on it to drag up or down.

"Centered Title" is one.

Other titling options are built into your Mac. They include all the text options in TextEdit, which lets you make lots of cool text. You convert the text into an image and import it.

You can also use the Ken Burns Effect in iMovie to zoom/pan across images of text. You can create a block of text of any style or combinations of style, capture it as an image, and import the image using the Ken Burns effect to animate it however you like.

It's possible too, of course, to purchase third-party title plug-ins for iMovie.

Karl